Electrocychc

From the colorless state it can be switched with light of short wavelength (A = 380 nm) via an electrocycHc ring opening and cis/trans rotation of one half of the molecule into a state with violet/purple color. The reverse reaction is effected by visible light (A = 580 nm). Since the system is metastable, one of the two reaction directions is matched by a rival thermal reaction, the thermoreversion. This progresses, however, in the case of benzospiropyran, at room temperature by a factor of 10 slower than the light-induced reaction. 

New photochromic dyes with electrocycHc reactions have been proposed on the basis of 1,5-electtocycHzation of heterogenous pentadienyl—anions (124). StiH newer are investigations into the photocycHzation of 2,4,6-tri-isoptopylbenzophenones for vinyl polymers ia the glassy state (133). 

A second process that occurs concurrently with the dissociation— redistribution process is an intermolecular rearrangement by which cyclohexadienone groups move along a polymer chain. The reaction maybe represented as two electrocycHc reactions analogous to a double Fries rearrangement. When the cyclohexadienone reaches a terminal position, the intermediate is the same as in equation 8, and enolization converts it to the phenol (eq. 9). 

Photochromism Based on Electrocyclic Reactions. The most common general class of photochromic systems involves reversible electrocychc reactions. Within this general class, the most weU-studied compounds are the indolino spiropyrans and indolino spiroxa2ines. 

Another class of photochromic compounds, which operate through an electrocychc mechanism, is the fulgides although, with this class, the colored species is formed through a ring formation rather than ring-opening (29,30). The reversion to the colorless species [59000-86-1], does not occur... 

The reaction of arc generated atoms with cyclooctatetraene (37) produces indene (22) in which the label is on C9. An initial DBA followed by ring opening to cyclononapentaene (38) is proposed. In analogy with known reactions of 38, electrocychc closure followed by H migration leads to 22- C9 (Eq. 25). 

Stoichiometrically protonated cycloheptadienones appear to undergo electrocycHc ring-opening/iing-closure processes R. Noyoii, Y. Ohnishi and M. Kato, J.Am. Chem. Soc., 94, 5105 (1972). 

SCHEME 22 Activation and reaction volumes of electrocycHc rearrangements... 

In real space, that is, in a chemical view, electrocycHc reactions are governed by the Woodward-Hoffmann rules of orbital symmetry conservation. According to Fukui only the frontier orbitals are regarded here to decide whether a given reaction is permitted or not The highest occupied orbital must be examined for a thermal process, which applies to the imaginary formation of a carbon nanotube from a graphene layer. (The first unoccupied orbital would have to be considered for a photochemical process-which is not the case here.)... 

The product is clearly derived from a [3 -I- 2] cycloaddition. Disconnection of the appropriate bonds leads back to an azomethine ylide intermediate, which can be formed by electrocychc ring opening of the aziridine. 

Acetylenes with electron-withdrawing substituents such as Me02C = CC02Me or NCC s CCN have a rich cycloaddition chemistry. As alkynyliodonium salts are highly electron-deficient acetylenes they are expected to undergo a variety of electrocychc processes. 

As related to Diels-Alder reaction this also represents a 6jr electron electrocychc reaction. Like Diels-Alder reaction the ene reaction is reversible. For example, the produce) 1-pentene of the ene reaction between ethene and propene, gives back ethene and propene on decomposition at 400° C. 

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